Method of roasting refractory manganese-precious metal ores



United States Patent METHOD OF ROASTIN G REFRACTORY MANGANESE PRECIQUSYMETALORES .Francis Aimone; Greenwich, arid Howar d Taba'ch-nick,

Stamford, :;Conn., assignors :to' "American sCyanamid Company, New York, N. Y a:corporafion1 of Maine 'No'Drawing. ApplicatiouDecemberao;I952,

Serial No.327;1'74

This invention.relates.to.the.treatment ofrefractory The refractory .nature .of .manganese ores containing gold and silver values-is.w.e1l.known.andiit is also :known that such ores when subjected :to :certain specific heat treatment 'operations .make -:the r precious metal Lcontent thereof :amenable .to .cyanidation.

It has heretofore .heen proposed :toiroast such-.ores in the presence of areductant asncokingicoal. While this treatment is eifectivein the case or some ores inincreasing the recoveryof the precious metal content" by a subsequent cyanidationstep,- other" ores do not respond well to this method of treatment. There remains, therefore, a need for an improved means for recovering precious metals in refractory combination with manganese permitting a high recovery of the precious metal content. In accordance with the present invention, we have discovered that a cellulosic-type material such as wood products and woody materials such as wood sawdust, green wood chips, small wood blocks, wood tar, etc. when used as a reductant in the roasting process effects a greatly increased recovery of the precious metal values, usually averaging better than 80% recovery. The mechanism that takes place when wood is used as a reductant in this type of reducing roast is not fully known and, therefore,

' the present invention is not limited to any theory of action. It is possible, however, that the volatile products resulting from the destructive distillation of the wood is responsible in some not well understood manner for reducing the manganese ore to a lower oxide so as to make the reduced ore especially amenable to cyauidation, with the result that the recovery of the precious metal content is increased considerably over that obtained when the reductants of the prior art are used.

In carrying out the present invention, it is necessary only to charge the ore to a suitable furnace, add thereto the desired quantity of cellulose material such as wood either in the form of wood chips, wood blocks, wood tar, sawdust, etc. and heat the charge in the absence of air to a temperature at which the wood is destructively distilled. The reduced ore is thereafter cyanided in the usual manner.

Roasting temperatures ranging upwards from about 400 C. have given satisfactory results. In general, however, it is rarely necessary to roast the charge much above 450 C., because at this temperature the volatile products from the destructive distillation of the wood have been driven off and which reduce the ore so as to make it amenable to subsequent cyanidation. The length of time the charge is heated is not unduly critical and need not be beyond the minimum time required to complete the reaction.

The amount of Wood used as a reductant in the roasting process is also not unduly critical, although sutficient wood must, of course, be present so that the required amount of volatile products are released to effectively re- I 'duce the .manganese minerals. vsatisfia'etory 'results have i been obtained whent-the 'weighnof (.1116? wood amounts 1 to fromt about 21% to about 110% tofxthe zweight-iofithe ore, depending on the type- .of z-woodxrand the manganese con- --tent .of .the sore.

vAftertthe chargerhas beeniheate'd sufiicien'tly- -to eifect .the desiredreductionpf the ore, 'the2'charcoal-is preferably -removed ibynscreenirig or iotherasuitableimeans zand re 10 -covered as 1 a by-prodnct. Fhexreduced 1'016 lxis 1 then ssubjected to cyanidation in the usual manner. *Any 'fine, abraded charcoal not removedz-prior to cyanidation may be recovered following cyanidation if. the charcoal is found .to contain adsorbed precious metal-values.

iIt is.'also within the scope of 't he,presentinvention,tp destructively distill the wood in a separate chamber and conduct all oftheflistillation products toftheffurnacefin which the oreis to'be reduced. 'This proceduregnay be preferable in cases wherethe danger of loss ofivalues by adsorption on charcoal presentsa problem.

The invention will be .desc'ribed greater .detail 1 in conjunction withtthejfollowing specific examples.

.-Examp l e :J A manganese-silver'ore assaying 51.2 oz. silver and 21. 4% manganesetwasjgroundjto 71.8% plus l0 0lmesh.

The ore was cyanided at a solution strength of between 0.075 and 0.1% "Na'CNjequivalent inlthe formjo'f commercial grad-e "calcium "cyanide, and 0.019% to 0.022% CaO for a period of 48 hours at a pulp density corresponding to 25% solids with the usual aeration. At the end of 24 hours of cyanidation 17.5% silver was recovered and at the end of 48 hours an additional 3.7% silver was recovered making a total silver recovery of only 21.2%.

Example 2 The same manganese-silver ore described in Example 1 was crushed to minus 10 mesh and charged to a rotary kiln-type furnace modified to exclude outside air. 10% by Weight of fine Wood dust was added to the charge and the charge was heated at 700 C. for 2 hours. The charcoal was not removed from the ore. The reduced ore was cyanided as described in Example 1. At the end of 24 hours of cyanidation, 71.3% silver was recovered and at the end of 48 hours an additional 10.3% silver was recovered bringing the total silver recovery to 81.6%.

Example 3 For comparative purposes, a second portion of this same ore was reduced at 700 C. with 10% by weight of coking coal and the residue cyanided as above. The total silver recovered after 24 hours of cyanidation amounted to only 36.3%.

Example 4 A refractory manganese-silver ore from a.third source crushed to minus 10 mesh and assaying 46.5 02. silver and 2.9% manganese was charged to the previously described furnace. 10% by weight of pine wood blocks about x x 1 /2" in size were added to the charge and the ore was reduced at 700 C. as in Example 2. The reduced residue was cyanided as in Example 1 and after 24 hours of cyanidation the silver recovery amounted to 58.9% and after 48 hours an additional silver recovery of 22.2% was obtained giving a total silver recovery of 81.1%.

Example 5 Another sample of the ore as described in the preceding 3 example. After 48 hours of cyanidation, the total silver recovery amounted to 78.8%.

Example 6 A refractory manganese-silver ore crushed to minus 10 mesh and assaying 27% manganese and 30.0 oz. silver was charged to a suitable furnace. by weight of pine wood tar was charged to the ore and the ore was reduced at 500 C. for 2 hours. The reduced residue was cyanided as in Example 1 and after 24 hours of cyanidation 62.6% silver was recovered and after an additional 24 hours of cyanidation an additional silver recovery of 14.2% was obtained, giving a total silver recovery of 76.8%. 1

Example 7 Another sample of the ore as described in Example 6 was charged to a suitable furnace. 5% of pine Wood blocks were added and the ore was reduced at 450-500 C. for 2 hours. The reduced residue was cyanided for 48 hours and gave a silver recovery of 77.2%.

We claim:

1. In the process of roasting refractory manganeseprecious metal ores in the presence of a reductant which serves to reduce the ores under non-chlorinating conditions so as to permit the recovery of the precious metal content thereof by cyanidation, the improvement which comprises forming a mixture consisting essentially of said ores and a reductant selected from the group consisting of wood and the wood tar resulting from the destructive distillation of wood, heating said mixture in the substantial absence of air at a temperature of from about 400 C. to about 700 C., the volatile products from said reductant serving to reduce said ores in a manner so as to make the precious metal content thereof more amenable to recovery by cyanidation.

2. The process according to claim 1 in which the manganese-precious metal ore is a manganese-silver ore.

3. The process according to claim 2 in which wood tar is used as the reductant in the reducing roast.

4. The process according to claim 2 in which pine wood is used as the reductant in the reducing roast.

5. The process according to claim 4 in which the reductant is present to the extent of from about 1% to 10% by weight of said ores.

References Cited in the file of this patent UNITED STATES PATENTS 43,016 Stevens June 7, 1864 52,120 Absterdam Jan. 23, 1866 205,536 Fryer July 2, 1878 229,509 Williams June 29, 1880 900,452 Wadhams et al Oct. 6, 1908 900,453 Wadhams et al Oct. 6, 1908 1,232,216 Caron July 3, 1917 1,234,426 Verrnaes July 24, 1917 1,864,222 Swainson et al June 21, 1932 2,200,563 Simpson May 14, 1940 2,234,378 Loring Mar. 11, 1941 FOREIGN PATENTS 531,118 Great Britain Dec. 30, 1940 635,904 Great Britain Apr. 19, 1950 

1. IN THE PROCESS OF ROASTING REFRACTORY MANGANESEPRECIOUS METAL ORES IN THE PRESENCE OF A REDUCTANT WHICH SERVES TO REDUCE THE ORES UNDER NON-CHLORINATING CONDITIONS SO AS TO PERMIT THE RECOVERY OF THE PRECIOUS METAL CONTENT THEREOF BY CYANIDATION, THE IMPROVEMENT WHICH COMPRISES FORMING A MIXTURE CONSISTING ESSENTIALLY OF SAID ORES AND A REDUCTANT SELECTED FROM THE GROUP CONSISTING OF WOOD AND THE WOOD TAR RESULTING FROM THE DESTRUCTIVE DISTILLATION OF WOOD, HEATING SAID MIXTURE IN THE SUBSTANTIAL ABSENCE OF AIR AT A TEMPERATURE OF FROM ABOUT 400* C. TO ABOUT 700*C THE VOLATILE PRODUCTS FROM SAID REDUCTANT SERVING TO REDUCE SAID ORES IN A MANNER SO AS TO MAKE THE PRECIOUS METAL CONTENT THEREOF MORE AMENABLE TO RECOVERY BY CYANIDATION. 